In order to meet recent exhausted gas regulations, improvement of purification performance of a catalyst is required. For example, in Patent Document 1, a plurality of catalysts is arranged in an exhaust chamber constituting a muffler. In addition, it is also considered to ensure the desired purification performance by increasing the size of the catalyst and the like.    Patent Document 1: Japanese Patent Application Publication No. 2016-113927 A
When the catalysts are arranged in the exhaust chamber as in Patent Document 1, exhausted gas after passing through the catalysts is all released to space in the chamber, and an output valley is easily formed at a specific engine rotation speed (for example, in a low-mid rotation speed range). In consideration of a flow of the exhausted gas, it is also considered to reduce the chamber volume such that the exhausted gas is easily directed toward the muffler, but it is assumed that muffling performance is reduced.